Liquid crystal display devices have been applied to, for example, watches, calculators, a variety of measuring equipment, panels used in automobiles, word processors, electronic notebooks, printers, computers, television sets, clocks, and advertising boards. Representative examples of types of liquid crystal display devices include a TN (twisted nematic) type, an STN (super twisted nematic) type, and a VA (vertical alignment) type and IPS (in-plane switching) type in which a TFT (thin film transistor) is used. Liquid crystal compositions used for such liquid crystal display devices need to satisfy the following requirements: being stable to external elements such as moisture, air, heat, and light; having a liquid crystal phase in a wide temperature range mainly including room temperature as much as possible; having a low viscosity; and enabling a low driving voltage. In addition, liquid crystal compositions are composed of several to tens of compounds to adjust, for example, dielectric anisotropy (Δ∈) and/or refractive index anisotropy (Δn) to be optimum to individual display devices.
A liquid crystal composition having a negative Δ∈ is used in vertical-alignment-type displays and widely used in, for instance, liquid crystal TV sets. Meanwhile, in all types of driving, there have been demands for low driving voltage, quick response, and a broad range of operation temperature. In other words, a liquid crystal composition having a positive Δ∈ with a large absolute value, a low viscosity (η), and a high nematic phase-isotropic liquid phase transition temperature (Tni) has been demanded. In order to determine Δn×d that is the product of Δn and a cell gap (d), the Δn of a liquid crystal composition needs to be adjusted to be in a range suitable for the cell gap. In addition, in the case where liquid crystal display devices are applied to television sets or other apparatuses, the liquid crystal display devices need to quickly respond, which generates a demand for a liquid crystal composition having a small γ1.
In traditional preparation of a liquid crystal composition having a small γ1, a compound having a dialkyl bicyclohexane skeleton is generally used (see Patent Literature 1). Bicyclohexane compounds, however, generally exhibit high vapor pressure, while they are highly effective for a reduction in γ1; in particular, such a tendency is especially remarkable in a compound having a short alkyl chain. In addition, such a compound also tends to have a low Tni; hence, an alkyl bicyclohexane compound having side chains with seven or more carbon atoms in total is employed in many cases in terms of the length of the side chain. A compound having a short side chain has not been sufficiently studied.
As liquid crystal display devices have come to be used in a broad range of applications, usage and manufacturing thereof have been greatly changed. In order to adapt to such changes, properties other than known basic physical properties need to be optimum. In particular, a VA (vertical alignment) type and an IPS (in-plane switching) type have become popular as liquid crystal display devices in which liquid crystal compositions are used, and display devices having a very large size (e.g., 50 inches or lager) have been practically used. An increase in the size of substrates has changed a technique for putting a liquid crystal composition between substrates, and a one drop fill (ODF) technique has become mainstream in place of a typically employed vacuum injection technique (see Patent Literature 2); however, dropping a liquid crystal composition onto a substrate generates droplet stains with the result that display quality is degraded, which has become problematic. Furthermore, in order to define the pretilt angle of molecules of a liquid crystal material in a liquid crystal display device and enable quick response, PS (polymer stabilized) liquid crystal display devices and PSA (polymer sustained alignment) liquid crystal display devices have been developed (see Patent Literature 3), and the above-mentioned disadvantage has become more problematic. In particular, such liquid crystal display devices are characterized in that a monomer is added to a liquid crystal composition and that the monomer in the composition is cured. Since liquid crystal compositions used for active-matrix devices need to constantly have a high voltage-holding ratio, limited compounds can be used for such compositions; thus, use of compounds having ester bonds is restricted. A monomer used in PSA liquid crystal display devices is mainly an acrylate-based monomer which generally has an ester bond; however, such a compound is not normally used as a liquid crystal compound for active-matrix devices (see Patent Literature 3). Such an unsuitable material causes droplet stains to be generated, and defective display leads to a reduction in a yield of a liquid crystal display device, which has been problematic. Moreover, addition of additives, such as an antioxidant and light absorber, to a liquid crystal composition also causes the problem of a reduction in a yield.
The term “droplet stain” is herein defined as a phenomenon in which a white stain of a droplet of a liquid crystal composition appears on a screen in a black-display mode.
A technique for reducing droplet stains is disclosed, in which a polymer layer is formed in a liquid crystal layer through polymerization of a polymerizable compound added to a liquid crystal composition to reduce droplet stains caused by an effect of an orientation control film (Patent Literature 4). In this technique, however, the polymerizable compound added to the liquid crystal composition causes a problem of screen burn-in, and an effect of a reduction in droplet stains is insufficient; hence, development of a liquid crystal display device which is less likely to suffer from screen burn-in and droplet stains while maintaining basic properties inherent in liquid crystal display devices has been demanded.